4Ts score for heparin-induced thrombocytopenia
Pretest probability of HIT.
Inputs
0 of 4Thrombocytopenia
Timing of the fall
Thrombosis or sequelae
Other causes of thrombocytopenia
When to use
- Thrombocytopenia during heparin exposure.
Pearls and pitfalls
- Low score (0–3): HIT very unlikely (negative predictive value about 99.8%); usually no testing.
- Intermediate or high: stop all heparin, start a non-heparin anticoagulant, and send an immunoassay.
Why use
Guides testing and treatment (ASH 2018).
| Criterion | Points |
|---|---|
| Thrombocytopenia: Fall below 30% or nadir below 10 × 10⁹/L | 0 |
| Fall 30–50% or nadir 10–19 | +1 |
| Fall above 50% and nadir 20 or higher | +2 |
| Timing of the fall: Before day 4 without recent heparin | 0 |
| Consistent but unclear, after day 10, or within 1 day with heparin 30–100 days ago | +1 |
| Day 5–10, or within 1 day with heparin in the past 30 days | +2 |
| Thrombosis or sequelae: None | 0 |
| Progressive or recurrent thrombosis, suspected thrombosis, or red skin lesions | +1 |
| New thrombosis, skin necrosis, or acute systemic reaction after heparin bolus | +2 |
| Other causes of thrombocytopenia: Definite | 0 |
| Possible | +1 |
| None apparent | +2 |